December 1946 Flying Magazine Advertisement for VHF Radios (large and small)
INTRODUCTION
With the end of WW2, many electronics manufacturers had their contracts canceled,
and suffered a sudden downturn in production. One company in the USA recognized
an opportunity for the growth of peacetime aviation, for personal aircraft and
commercial airliners. They produced prototype radios for each possibility.
The radios used in early WW2, were intended for communications, mainly in the
HF frequencies, and navigation, mainly in the LF frequencies. Towards the end
of WW2, there was a trend towards VHF for communications. The company, Aircraft
Radio Corporation (Boonton, New Jersey, USA), had designed the original prewar
ATA and ARA radios for the US Navy. These radios were then adopted as the SCR-274N
radios for the US Army Air Force. The Aircraft Radio Corporation then improved
these radios and they became the AN/ARC-5 radios for the US Navy. The post war
development of these same radios, was called the Type-12 series of radios,
which was later adopted by the military as the AN/ARC-60 series.
December 1946 Flying Magazine Advertisement for VHF Radios (large and small)
The top part of the advertisement shows a VHF transmitter and receiver from the World War 2 AN/ARC-5 series, (probably a T-89 or T-90 and R-112 or R-113), suggested for a Lockheed Constellation airliner. The lower part shows the Type-12 VHF transmitter and receiver (probably a T-11 and R-15), suggested for a private aircraft.
To avoid confusion, note that A.R.C. is an abbreviation of the Aircraft Radio Corporation. Also note that ARC is the Joint Army Navy (JAN) nomenclature for Aircraft Radio Communication. So this series of radios is the A.R.C. Type-12, and also the AN/ARC-60. Not all Type-12 radios were adopted in the ARC-60 series. Also note that the same radio may have a Type-12 nameplate, or an ARC-60 nameplate. I use ARC-60 and Type 12 interchangeably. I use Kcs and Mcs for the older equipment and MHz for actual measurements.
The ARC-60 was used in the Korean War, and consisted of 5 receivers and 2 transmitters. They were intended for light aircraft, but were adopted for helicopters and small planes. There is another A.R.C. series of radios, similar looking in size, colour and shape, which provide Automatic Direction Finding (the Type-21 or AN/ARN-59), and a Glide Slope receiver (the Type-15 or AN/ARN-30). The Type-15 and Type-21 will not be discussed here.
| Function | Characteristics | Type-12 | ARC-60 |
|---|---|---|---|
| Receiver | 520 - 1500 Kcs | R-10A (14V) | - |
| Receiver | 520 - 1500 Kcs | R-10A (28V) | - |
| Receiver | 190 - 550 Kcs | R-11A (14V) | R-510/ARC |
| Receiver | 190 - 550 Kcs | R-11A (28V) | R-511/ARC |
| Receiver | 108 - 135 Mcs | R-15 (14V) | - |
| Receiver | 108 - 135 Mcs | R-15 (28V) | R-509/ARC |
| Receiver | 118 - 148 Mcs | R-19 (14V) | R-507/ARC |
| Receiver | 118 - 148 Mcs | R-19 (28V) | R-508/ARC |
| Receiver | 75 Mcs | R-20 (14V) | - |
| Receiver | 75 Mcs | R-20 (28V) | - |
| Dynamotor | 14V to 250Vdc | D-10 (14V) | DY-91/ARC |
| Dynamotor | 28V to 250Vdc | D-10 (28V) | DY-90/ARC |
| Dynamotor | 14V to 250Vdc | D-10A (14V) | DY-89/ARC |
| Dynamotor | 28V to 250Vdc | D-10A (28V) | DY-86/ARN-30 |
| Transmitter | 116 - 132 Mcs | T-11A (14V) | T-365/ARC |
| Transmitter | 116 - 132 Mcs | T-11A (28V) | T-366/ARC |
| Transmitter | 116 - 132 Mcs | T-11B (14V) | T-365A/ARC |
| Transmitter | 116 - 132 Mcs | T-11B (28V) | T-366A/ARC |
| Transmitter | 132 - 148 Mcs | T-13 (14V) | T-364/ARC |
| Transmitter | 132 - 148 Mcs | T-13 (28V) | T-363/ARC |
| Transmitter | 132 - 148 Mcs | T-13A (14V) | T-364A/ARC |
| Transmitter | 132 - 148 Mcs | T-13A (28V) | T-363A/ARC |
| Transverter | 228 - 258 Mcs | TV-10 (14V) | - |
| Transverter | 228 - 258 Mcs | TV-10 (28V) | - |
| Antenna | VHF | A-12 | AT-383 |
| Antenna | VHF | A-15 | - |
| Antenna | VHF | A-16 | - |
| Antenna | Loop | L-10 | AT-382 |
| Antenna | Loop | L-10A | AT-382 |
| Relay | Muting | K-11 | - |
| Relay | Muting | K-12 | - |
| Relay | Oscillator | K-13 | O-423/AR |
| Mounting | Transmitter | M-11A | MT-1142/ARC |
| Mounting | Receiver and Transverter | M-12A | MT-1140/ARC |
| Mounting | Receiver | M-23 | - |
| Junction Boxes | Many | Many | |
| Control Boxes | Many | Many | |
| Connectors | Many | Many |
The main radios are two VHF transmitters and matching receivers, for AM voice communications, 108-135 Mcs and 118-148 Mcs, for aircraft. There are also two Navigation receivers, covering 190-550 Kcs and 520–1500 Kcs. The radios were available as 12 volt or 24 volt DC versions. There was one beacon receiver on a fixed frequency of 75 Mcs. There was one transverter that provided transmit and receive access to the military UHF band. The TV-10 transverter and R-20 beacon radios will not be discussed here.
MECHANICAL DESIGN
They are very similar to the ARA/ATA, SCR-274N and ARC-5 series of radios
from WW2. The receivers are the same physical size and shape, and use many
of the same metal stampings. The chassis are a molded aluminium deep channel,
with a front panel, and stamped metal covers on the top and bottom, all riveted
together. They are very nicely made. They use many small nickel plated screws,
to fix parts underneath. The covers are held on by similar screws but these
are black, so that you know which ones to remove for access.
Construction: Front View
On the receivers, behind the front panel is the tuning capacitor, valves and IF transformers on the top, with the dynamotor at the rear. Underneath the chassis are the RF coils, and tub encapsulated capacitors. There is an extensive use of mica insulators, and tag strip mounted resistors. Whereas the ARA, ARC-5, and SCR-274N receivers were painted black wrinkle, these are painted a distinctive grey wrinkle colour.
Construction: (bottom view)
They are a remote controlled receiver, so there is no need for any local controls or a tuning dial. The early models of the Type-12 used an ARC-5 style mounting rack, with a rear connector, but the later more numerous models had the connectors on the front only. This simplified the mounting rack, which became just a physical support and a shock mounting. The transmitters are the same width and height as the receivers, but only half as long.
Connectors (left ARC-60, right ARC-5)
The ARC-5 connectors used a black Bakelite insulator, and a locating screw (which earthed one of the silver plated pins). The ARC-60 connectors looked the same, but they use a white ceramic insulator held in by a cir-clip, thus eliminating the need for a compulsory earth pin. These pins are gold plated. The radios can be configured in many different arrangements. There are several different remote control boxes, and tuning is by mechanical Bowden cable. The navigation receivers can use a manual rotating loop for direction finding.
ELECTRICAL DESIGN
The receivers are a super-heterodyne type, and use a combination of miniature
valves, loctal valves, and octal valves, and are continuously tunable, using a
dynamotor as the high voltage power supply. None of these radios have a BFO,
as they are intended for AM reception and transmission only.
The older dynamotor was a D-10, rated at 14V and 2.8A input (or 28V and 1.4A). The newer dynamotor was a D-10A, rated at 14V and 3.4A input (or 28V and 1.7A). The output is quoted at 250 V and 0.85A. It is noted on the circuit diagrams that the voltages are shown differently. The circuits show that the R-10A and R-11A is 281V or 262V, and the R-15 and R-19 is 277V or 262V.
D-10A Dynamotor 28 volt and 14 volt
The IF transformers are wired in place, rather than using plug in sockets, like the previous ARC-5 radios. Some thought has gone into the radios, as a total system. There are different component numbers used on the circuit diagrams. In the receiver R-10A all components are numbered in the 500 range, R-11A uses the 600 range, R-15 the 100 range, R-19 the 300 range, R-20 the 2700 range, T-11A the 200 range, T-11B the 2300 range, T-13 the 400 range, T-13A the 2400 range, and TV-10 the 4200 number range. This probably gives an indication of the sequence in which they were designed.
The transmitters use miniature valves (type 6AQ5 or 5673) and produce 2 watts, with AM modulation. There are 5 crystal locked frequencies. The early models had a tuning meter on the front panel, then it was changed to a test point on the front panel, but the more common later ones, used an internal test point. These require an external meter.
Dynamotor in place
RECEIVER TYPE R-10A
The R-10A is very similar to the WW2 ARC-5 receiver model ARC-5/R-24,
covering 520 to 1500 Kcs. (The ARC-60 did not use this model.) The R-10A is
mainly intended as a navigation receiver, and can be used for direction finding
or homing on broadcast signals. It is a 6 valve super-heterodyne receiver,
using loctal and octal valves. It has an IF of 239Kcs and uses part numbers
in the 500 range. It has a single post terminal (C-501) for the aerial input,
and a BNC connector for a loop aerial input (J-501). A relay switches between
them. There is a knob on the front panel which is an aerial trimmer. The RF
amplifier is a 14A7 loctal based pentode, with RF gain control on the cathode
and AGC on the grid. The signal then goes through an RF transformer to the 14S7
loctal mixer. It has AGC on the grid. This valve is also the local oscillator.
It uses a 3 gang tuning capacitor. The signal then goes through an IF transformer
to the first IF amplifier, a 14A7 loctal pentode, with RF gain control on the
cathode and AGC on the grid. The signal then goes through another IF transformer
to the second IF amplifier, a 14R7 loctal pentode. This valve has no RF gain control
or AGC control. This valve contains a diode used for AGC rectification. The signal
then goes through the third IF transformer to the detector, which is a loctal 14F7
dual triode. One triode is connected as a diode to perform AM detection. The
second triode is connected as a noise limiter. The audio signal then goes to a
12A6 octal power pentode for amplification. This goes to an output transformer.
On the top of the chassis at the rear, is a 3 pin socket for a plug in dynamotor.
A 14 volt or 28 volt dynamotor can be plugged in, which produces 250 VDC.
The valve heaters are wired in parallel for 14 volt operation, for the R-10A(14V) or the valve heaters are wired in series parallel for 28 volt operation, for the R-10A(28V).
There are 3 sockets on the front panel, and none on the rear. The 2 pin plug (J-502) is for power input. The 6 pin plug (J-503) is for remote control operation. A control box C-16 (or similar) can be used, with the appropriate frequency dial fitted. The other 6 pin plug (J-506) in the top center location is only used when a transmitter is connected. For single receiver operation, 2 pins are bridged, so that the dynamotor power is always connected to the receiver. When used with a transmitter, this plug is connected to the transmitter, and the dynamotor powers the transmitter. The older ARC-5/R-24 had socket on the rear for power and remote control. The R-24 also had a plug in adapter on the front for remote or local control operation. This radio does not.
R-10A Front
R-10A Back
R-10A Top
R-10A Bottom
R-10A Tuning Capacitor
R-10A Block Diagram
R-10A Circuit
C-16 Remote Control
R-10A Wiring
RECEIVER R-11A (R-510, R-511)
The R-11A is very similar to the WW2 ARC-5 receiver model ARC-5/R-23, covering 190 to 550 Kcs. The R-11A is mainly intended as a navigation receiver, and can be used for direction finding or homing on beacon signals. It is basically the same as the R-10A. It has an IF of 85Kcs and uses part numbers in the 600 range. All the radios are that I have are made by A.R.C. except this one, which is made by Stromberg-Carlson.
R-11A Front
R-11A Block Diagram
R-11A Circuit
R-11A Wiring
RECEIVER R-15 (R-509)
The R-15 is a receiver covering 108 to 135 Mcs. This is mainly intended
as a communications receiver. It is a 9 valve super-heterodyne receiver, using
miniature, loctal, and octal valves. It has an IF of 15Mcs and uses part numbers
in the 100 range. It has a single BNC connector for the aerial input (J-106).
The first and second RF amplifiers are 9003 pentodes (B7G base), with RF gain
control on the first cathode and AGC on both the grids. The cathodes are unbypassed.
The signal then goes to the 9003 pentode mixer (B7G base). Note that there is an
error in the circuit, as the mixer anode appears unconnected. There is a wire
missing on the circuit. The local oscillator is a 9002 triode (B7G base). The
oscillator has a circular earthed shield, which adds capacity and lowers the
frequency. The output is injected into the mixer cathode. These 4 valves, the
4 gang tuning capacitor and tuned circuits, are on a separate sub chassis behind
the front panel. The signal then goes through an IF transformer to the first IF
amplifier, a 14A7 loctal pentode, with RF gain control on the cathode and AGC on
the grid. The IF transformers are called “coupling units” on the circuit diagram.
The signal then goes through a second IF transformer to the second IF amplifier,
a 14R7 loctal pentode, with AGC on the grid. One of the diodes is connected to the
AGC line. The signal then goes through a third IF transformer to the third IF
amplifier, a 14R7 loctal pentode. One of the diodes is the AGC rectifier. The
signal then goes through the fourth IF transformer to the detector, which is a
loctal 14F7 dual triode. One triode uses the grid as a diode, to perform AM
detection, the plate and cathode are connected together. The second triode is
connected as an audio amplifier. Due to careful selection of DC levels by R-142
and R-143, the audio amplifier is close to cut off. When there are large noise
spikes, the potential at R-140 and R-141 causes cut-off, and the amplifier acts
as a noise limiter. See paragraph 2-40 in the manual (USAF) T.O. 12R2-4-1-2 10
January 1952 for a detailed explanation of this function. There is a preset
control on the front panel R-15 that is labeled SQUELCH. The audio signal then
goes to a 12A6 octal power pentode for amplification. The anode goes to an output
transformer. The cathode is connected to the control box, and an extra resistor
can be switched in to provide HI or LO audio output. On the top of the chassis
at the rear, is a 3 pin socket for a plug in dynamotor. A 14 volt or 28 volt
dynamotor can be plugged in. The valve heaters are wired in series parallel for
the different voltage operation.
There are 3 sockets on the front, and none on the rear. The 2 pin plug (J-101) is for power input. The 6 pin plug (J-102) is for remote control operation. A control box C-17 (or similar) can be used, with the appropriate frequency dial fitted. The other 6 pin plug (J-103) in the top center location is only used when a transmitter is connected. For single receiver operation, 2 pins are bridged, so that the dynamotor power is always connected in the receiver. When used with a transmitter, this plug is connected to the transmitter, and the dynamotor powers the transmitter.
R-15 Front
R-15 Top
R-15 Tuning Capacitor
R-15 Earthed Shield
R-15 Bottom
R-15 Block Diagram
R-15 Circuit
C-17 Remote Control
R-15 Wiring
RECEIVER R-19 (R-507, R-508)
The R-19 is a receiver covering 118 to 148 Mcs. This is mainly intended
as a communications receiver, and is basically the same as the R-15. It has
an IF of 15Mcs and uses part numbers in the 300 range.
R-19 Front
R-19 Block Diagram
R-19 Circuit
R-19 Wiring
RECEIVER ONLY INSTALLATION
Wiring up a single receiver is fairly simple. The 28 volts input is
connected to the 2 pin connector on the front of the receiver. The lower
right hand connector is connected to the control box. The top center connector
is not used. It has a link between pin A and pin E, to make the HT available
to the receiver, and it is normally covered by a screw on dust cap.
A mechanical Bowden cable provides receiver tuning. The VHF receivers
have a BNC connector for the antenna to be directly connected to the receiver.
The LF and BC receiver have a terminal for a long wire antenna, and a BNC
connector for a loop antenna. There is a small knob on the receiver front
panel that can be adjusted to peak the antenna input. Headphones are plugged
into the phone jack (TEL) on the receiver front.
Dust Cap and Link
The control box has a knob which is the RF gain control, and also switches the radio on. There is also a dial calibrated with the receiver frequency. It has a crank that rotates the dial and connects to the receiver tuning capacitor through the Bowden cable. In the centre of the dial is a switch. On the LF receivers, it can switch between the loop and the antenna terminal. It is labeled ANT/LOOP. On the VHF receivers, it can switch between low and high audio output. It is labeled LO/HI.
TRANSMITTER NUMBERING
The numbering of the transmitters is slightly confusing. Basically there
are only two transmitters. One covers the frequency range 116 - 132 Mcs,
and the second covers the frequency range 132 - 148 Mcs. Each of these came
in two versions, the first version uses 6AQ5 valves, and the later version
uses 5763 valves. So that is 4 different transmitter types. And each of these
came in 14 volt and 28 volt versions, which means 8 different transmitters.
The Type-12 numbering system has a T-11A and T-11B type, but the T-13 has a
non-letter and an A type (and 14 and 28 volt versions as well). The ARC-60
numbering is a little more sensible, having a different number for the 14 and 28
volt versions, and a non-letter for the original version and an A suffix for the
newer version.
| TYPE-12 Number | ARC-60 Number | Frequency | Valve |
|---|---|---|---|
| T-11A (14V) | T-365/ARC | 116 - 132 Mcs | 6AQ5 |
| T-11A (28V) | T-366/ARC | 116 - 132 Mcs | 6AQ5 |
| T-11B (14V) | T-365A/ARC | 116 - 132 Mcs | 5763 |
| T-11B (28V) | T-366A/ARC | 116 - 132 Mcs | 5763 |
| T-13 (14V) | T-364/ARC | 132 - 148 Mcs | 6AQ5 |
| T-13 (28V) | T-363/ARC | 132 - 148 Mcs | 6AQ5 |
| T-13A (14V) | T-364A/ARC | 132 - 148 Mcs | 5763 |
| T-13A (28V) | T-363A/ARC | 132 - 148 Mcs | 5763 |
TRANSMITTER T-11B (T-365A, T-366A)
This is a small transmitter covering the frequency range of 116 - 132 Mcs,
with 5 crystal locked channels. It delivers 2 watts AM, and uses 4 valves.
There are no switched tuned circuits, so the output frequency can only be in
a 2 Mcs wide band, anywhere in the frequency range. The components are numbered
in the 2300 range.
One of the 5 crystals is selected by several relays, selected from the remote control box. The 5763 valve (V2301) is a Pierce oscillator, which acts as a doubler. It is designed to oscillate at a twelth of the output frequency. There is a tuned circuit in the anode, tuned to double the crystal frequency. The crystal would need to be in the range 9.666 - 11.0Mcs. The signal is then capacitively coupled to another 5763 (V2302), set up as a tripler. There is another tuned circuit, at six times the crystal frequency. Then the signal is capacitively coupled to a final 5763 (V2303) which is the PA. It is also doubled here. The tank circuit is in the range 116 - 132 Mcs. Alternatively, the oscillator can also be a tripler, and operate at an eighteenth of the output frequency. The crystal would need to be in the range 6.444 - 7.333 Mcs. There is an aerial change over relay which switches the antenna between the PA and the receiver. The relay also switches the high voltage power supply (250 volts) between the transmitter and the receiver. There is a modulation transformer in the anode supply to the PA. This is driven by a 5763 valve (V2304). Its grid is connected to a microphone transformer which comes from the front panel. This is for a carbon microphone. The PTT on the microphone energises the receive/transmit relay.
There is a diode from the antenna line, to a capacitor and resistor, at a test point inside the transmitter, called J-2301. This rectifies some of the RF output and a meter may be connected to measure the DC voltage here, so that the transmitter can be aligned. The early models had a meter on the front panel.
The microphone and modulator amplifier is connected to the receiver, so that it can supply a sidetone. The remote control box has a sixth position (after the 5 crystal positions) which is labeled INT. There is no crystal associated with this switch position, and the microphone and modulator amplifier are connected to the receiver, so that it acts as an intercom. The 28 volt transmitters have the heaters connected in a series, and the 14 volt transmitters have the heaters connected in a series/parallel arrangement. The T-11A and T-11B transmitters are almost the same, except the valves inside are all 6AQ5 instead of all 5763. There are minor component differences. The 6AQ5 heaters require 1A at 14 VDC, or 0.5A at 28VDC. The 5763 heaters require 1.5A at 14 VDC, or 0.75A at 28VDC. The sidetone is picked off the plate in the 6AQ5 and from the cathode of the 5673. All the components have a 200 numbering system for the T-11A and a 2300 numbering system for the T-11B. The J-2302 connector is normally connected to the J-103 connector on the R-15 receiver. The receiver would then supply the 250 VDC and the 28 VDC. The transmitter will supply sidetone to the receiver. The J-2303 connector is connected to the channel switch on the control box C-13. A microphone would be plugged into the microphone jack J-2307 labeled MIC. J-2306 is another connector (4 pins) and it duplicates the microphone jack. The antenna and receiver is attached to the 2 BNC connectors, one marked TO ANT and the other marked TO REC.
T-11B front
T-11B back
T-11B top
T-11B underneath
T-11B block Diagram
T-11B circuit
Transmitter Control Box C-13
Transmitter Control Wiring
TRANSMITTER T-13A (T-364A, T-363A)
This is a small transmitter covering the frequency range of 132 - 148 Mcs,
with 5 crystal locked channels. It delivers 2 watts AM, and uses 4 valves.
There are no switched tuned circuits, so the output frequency can only be in
a 2 Mcs wide band, anywhere in the frequency range. It is similar to the
T-11B transmitter. All the components have a 400 numbering system for the
T-11A and a 2400 numbering system for the T-11B.
The crystal would need to be in the range 11.0 - 12.333 Mcs. Alternatively, the oscillator can also be a tripler, and operate at an eighteenth of the output frequency. The crystal would need to be in the range 7.333 - 8.222 Mcs.
This transmitter is in very good condition, and appears to be little used. The front panel of my example is engraved with 2400 series numbers, but has been over sprayed with grey paint, and new 2300 series numbers have been stenciled in white. This is incorrect! It also has wires to hold the top clips closed.
T-13A
The transmitter frequency range can be moved down by fitting a frequency plate. This is a metal plate that mounts in the transmitter and adds extra capacity around the valves. This can change the frequency range of the transmitter from 132 - 148 Mcs to 125 - 140 Mcs.
Capacity Plate
Capacity Plate Fitted
T-13A Block Diagram
T-13A circuit
Transmitter Control Box C-13
Transmitter Control Wiring
RECIEVER AND TRANSMITTER INSTALLATION
Wiring up a transmitter is a little more complicated. The transmitter cannot be
used by itself, as it is powered from the receiver.
The transmitter control box has only one control. It is a switch to select one of the crystal controlled channels. A microphone is plugged into the transmitter microphone jack, and its PTT button activates the transmitter and mutes the receiver. The receiver top connector is connected to the transmitter LHS connector, as the receiver dynamotor provides HT and LV for the transmitter. The receiver antenna goes to the transmitter and uses the antenna changeover relay. A transmitter control box is connected to the RHS transmitter connector, to allow the channel to be selected. The top right connector is not used, and is usually covered by a cap. If desired, the microphone and PTT can be connected through the top RHS connector instead of the microphone jack. The C-13 control box has a transmitter ON/OFF switch which can be wired in if required. Most other boxes do not have this.
RECIEVER AND TRANSMITTER INSTALLATION (with NETTING)
It would appear that the use of a crystal controlled transmitter would be
simple to operate, only requiring the receiver to be tuned to the dial reading of
the transmitter frequency, or to another transmitter on that frequency. When used
at an airport, this would probably be the airport control tower.
However, there may have been an additional requirement to add a NET facility. This would be useful, when there was no external transmitter to tune to, or when "radio silence" was necessary. This required another series of control boxes, different to all the previous ones. These control boxes, incorporate the netting facility, called the "WHISTLE THROUGH" function. This was operated by pressing the receiver tuning crank handle IN. It was only a minor modification to the control box, but it required the receiver control box to be replaced, when netting was needed. It also required some harness rewiring and an Oscillator K-13 (O-423/AR) to be fitted. One oscillator was required for each receiver transmitter pair.
OSCILLATOR K-13 (O-423/AR)
K-13 Oscillator
K-13 Oscillator underneath
This is small box with one 24 pin connector and 2 BNC connectors. It controls several transmit and receive functions. It is operated by pressing the tuning crank on a VHF receiver control box, for either the R-15 or R-19 receivers. All components have a 4000 series numbering.
There are 5 relays inside the box. They all operate together, when the crank is pressed in. Relay K4001 disconnects the receiver Sensitivity control, and using relay K4005 connects the receiver to a preset sensitivity control inside the K-13 box. There are 2 presets, one for a UHF receiver, and one for a VHF receiver (labeled WHISTLE LEVEL). Relay K4002 keys the transmitter ON, and switches the audio output. Relay K4003 connects HT to both receiver and transmitter simultaneously. It also starts a "relaxation" oscillator, which generates a 1khz audio tone for the transmitter microphone input. This is just a neon lamp, a resistor and a capacitor. Relay K004 connects the transmitter ouput to a dummy load inside the K-13 box, to prevent RF radiation. This oscillator example appears new, and is painted in smooth grey paint, not grey wrinkle paint.
K-13 Oscillator Circuit
Wiring for Netting using the K-13 Oscillator
CONTROL BOXES
Each receiver control box consists of an ON/OFF switch and a SENSITIVITY control.
There is a rotary dial and a Bowden cable drive for frequency tuning. In the centre
of the dial is another switch, which selects the ANT/LOOP relay on the navigation
receivers, or audio HI/LO level on the VHF receivers. The tuning dials can be changed
to show the appropriate frequency. The AZIMUTH control for rotating the loop is a
Bowden cable drive using 360 degree dial. The transmitter control box is merely a
rotary switch for selecting the channel crystal.
The control boxes are a small aluminium pressed tub, painted wrinkle grey. These are mounted on a convenient bulkhead. Later model control boxes were made in square black boxes, for mounting in an aircraft instrument panel. There appears to be an electrical change, as the low numbered boxes were labeled SENSITIVITY and used a 50K potentiometer on the RF gain line. Other control boxes were labeled VOL. and used a 5K potentiometer for audio level control. These coincided with the addition of the WHISTLE THROUGH switch.
As the system can be configured with 1 to 4 receivers and several transmitters, there were individual control boxes for each transmitter and receiver, or boxes that had multiple controls in one unit. Because of this flexibility, there are many control boxes and permutations. The square instrument panel control boxes, were usually individual controls for individual receivers and transmitters. The manuals show the list of control boxes with many gaps. It appears that the control boxes were improved, and low numbered boxes became obsolete.
ANTENNAS
Five antennas are used in the ARC-60 or Type-12 series. Antenna ARC-12296
is a long wire antenna kit for LF and broadcast reception. Antenna A-12 is a
quarter wave vertical VHF antenna, intended for aircraft speeds up to 200 mph.
Antenna A-15 is a quarter wave inverted L shaped antenna, covering 116-148 Mcs,
and aircraft speeds up to 250 mph. Antenna A-16 is a quarter wave inverted L shaped
antenna, covering 228-258 Mcs, and aircraft speeds up to 500 mph. Antenna L-10A
is a rotatable 9 inch loop antenna for aural direction finding, using the C-18
azimuth control and a Bowden cable, for aircraft speeds of less than 200 mph.
L-10A loop
MOUNTS
The mountings are merely a sheet of aluminium with 4 shock mounts. The receiver
and transmitter clip in to these. The M-12A fits all the receivers and the TV-10.
The M-11A fits all the transmitters. The beacon receiver used the M-23 mount. Each
grey coloured control box has a mount. The black control boxes were fitted directly
in the instrument control panel and required no dedicated mouting.
Shock Mountings, M-12A receiver, M-11A transmitter, M-18 control box.
ASSEMBLING A SET
I assembled an ARC-60 or Type-12 set for 2 meter amateur use, and a field day
display. A sheet of 5 mm thick aluminum was cut to size to enable it to slide into
a 19 inch rack shelf, so it could also be used at home. This allowed space for two
receivers, one transmitter, an oscillator, and some control boxes across the front
edge. Three shock mounts were screwed to the base. A control box was mounted on the
front. A C-36 would have been the best control box (2 receivers), but I only had
a C-24 control box (3 receivers), so I used that.
Base, Shock mounts, Control box, and Bowden cables
I used an R-10A broadcast band receiver (520-1500 Khz), an R-508 receiver (118-148 MHz), a T-13A transmitter (132-148 MHz), an O-423/AR oscillator, a C-24 control box (3 receiver, 1 transmitter, azimuth control), an L-10A loop antenna, two M-12A shock mounts, one M-11A shock mount, three Bowden cables, and 7 connectors. The cabling was wired up and tested piece by piece. When finished it was laced into 2 neat looms. A microphone and headphones were attached. The 28 VDC power supply was external. There are only 2 small dynamotors, and the valve filaments to heat, so the power required was quite modest, 5 Amps at 28 volts. No restoration was necessary, just a little maintenance and some repairs. All radios were cleaned and tested. The dynamotor bearings were greased, and the brushes checked. The R-10A receiver had a shorted HT capacitor which was replaced. The R-11A receiver had a noisy tuning capacitor, so the bearings and wipers were cleaned. The R-15 receiver had no faults. The R-19 receiver had an HT short circuit, which turned out to be an amateur modification. It was reversed to what it should be. The receiver had extremely low sensitivity. It was dismantled, and the resistor behind the aerial connector was burnt to a crisp and the coax was blown to pieces, possibly due to a lightning strike. The resistor and coax were replaced. The gain was better but still low. One RF amplifier valve was gassy and was replaced. The receivers now all worked, and the sensitivity of them all was between 1 and 3 microvolts, for a 10 dB S/N. A crystal was ordered such that the T-13A could use the 2 meter amateur band. I chose 144.125 MHz for AM operation. This required an 8.006944 MHz crystal. The transmitter was aligned and the power output was found to be below specification. The metering diode CR2401 was shorted and loading the output, so it was replaced. It was noted that this transmitter looked brand new, so this may have been an old fault. The C-24 control box does not have WHISTLE THROUGH capability, so the HI/LO switch in the middle of the VHF dial, was used.
Type-12 Set Up
C-24 Control Box (the Range controls are not used in this setup)
CONCLUSION
The Type-12 radio setup works quite well.
The broadcast band receiver tunes easily, the Bowden cable drive is a little stiff and has a small amount of backlash, but this is not a problem on the broadcast band. The VHF receiver tunes easily as well, as the 15MHz IF is quite broad. Several amateur signals could be heard on the 2 meter band, and even FM signals could be copied. Both the receivers are sensitive. When the Netting function was switched on (using the HI/LO switch), the transmitter was easily found, due to its unique signal. Several amateur contacts were made, despite the low 2 watt output.
Due to the modular design, the radio can be configured in many different arrangements. The drawback, is that it requires many modules, each with associated wiring and connectors.
REFERENCES
ARC-12_equip_1_55, Instruction Book for A.R.C. Type 12 Equipment with UHF Supplement, Jan 1955
TO 12R2-4-1-2, Handbook of Maintenance Instructions, Radio Set ARC Type 12, 5 June 1956
TM 11-525-10, Operating Instructions for Radio Set ARC Type 12, March 1958
Copyright
Ray Robinson